In recent years, there has been a growing concern about environmental issues, which has led to a progress in conversion of the energy resources from fossil fuels such as petroleum and coal to alternative energy resources. Among these alternative energy resources, cells have been focused. Particularly, secondary cells which can be repeatedly charged and discharged are more used not only for electronic devices such as cell phones and laptops but also in other various fields such as cars and aircraft, and thus various secondary cells and the materials used for secondary cells have been researched and developed. Particularly, large-volume, light-weight lithium ion cells are a secondary cell the use of which is most expected to be expanded and are a cell that is most actively researched and developed.
As to a material used for such a secondary cell, the following compounds have been disclosed. Patent Literature 1, for example, discloses an ethylene oxide copolymer with a specified crystallization temperature and a glass transition temperature; and a polymer composition that contains an ethylene oxide copolymer and an organic metal salt. Patent Literature 2, for example, discloses a nonaqueous electrolyte obtained by dissolving an electrolyte in a nonaqueous solvent that contains 0.05 to 30 wt % of a polyalkylene glycol derivative with a specific structure. Patent Literature 3, for example, discloses a solid polyelectrolyte constituted by a solid solution of an ionic compound in a solid cross-linked polyether that is obtained by copolymerizing a vinyl ether having a specific structure and a divinyl ether having a specific structure and has a specific weight average molecular weight and a specific glass transition temperature. Patent Literature 4, for example, discloses the following ionic conductive materials each of which contains a polymeric acid lithium salt with a specific Li equivalent weight, and has a specific minimum pKa when both ends of a repeated structure of the polymeric acid lithium salt are replaced with methyl groups and lithium cations of the polymeric acid lithium salt are replaced with protons: namely, an ionic conductive material having a structure in which the lithium salt is in a side chain, and the main chain and the lithium salt in a side chain are bonded via a carboxyl group or a carbonyl group; an ionic conductive material having a structure in which a hydrocarbon group is in a side chain having lithium sulfonate at an end; and an ionic conductive material having a structure in which lithium carbonate or lithium sulfonate is directly bonded to the main chain. Patent Literatures 5 to 7, for example, each disclose a polymer solid electrolyte that contains an alkali metal salt and a comb polymer obtained by reacting a polyether having a reactive double bond with a structure that has a polyether diol in which hydrogen of a hydroxyl group at one end is replaced by a methyl or ethyl group, and hydrogen of a hydroxyl group at the other end is replaced by a double bond. Patent Literature 8, for example, discloses a solid, polymeric electrolyte in a membrane form, constituted by a solid solution of an ionic compound in a crosslinked polyether.